This invention relates to electrostatography and more particularly to a new and improved composition and process for developing an electrostatic image.
Electrostatography is exemplified by the basic electrophotographic process taught by C. F. Carlson in U.S. Pat. No. 2,297,691, which involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting latent electrostatic image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner". The toner is normally attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image, which may then be transferred to a support surface, such as paper. The transferred image is generally permanently affixed to the support surface by heating, although other suitable fixing means, such as solvent or overcoating treatment, may be substituted for the foregoing heat fixing step.
One method for applying the electroscopic material to a latent electrostatic image is the "cascade" process disclosed by L. E. Walkup in U.S. Pat. No. 2,618,551 and E. N. Wise in U.S. Pat. No. 2,618,552. In the cascade process, the developer is basically a two component system comprised of a finely divided electroscopic material (toner) and a carrier. The carrier is generally comprised of a core material, such as glass or sand, and a triboelectric resinous coating with the toner particles being loosely held on the coating by triboelectric action. The carrier and toner are chosen so that the toner assumes a charge of an opposite polarity to the latent electrostatic image, whereby upon rolling or cascading the developer over the surface bearing the electrostatic image, the toner particles are attracted to the image being developed. Conversely, if the toner particles have a charge of identical polarity to the latent electrostatic image, the toner particles accumulate on the background portions.
In automatic electrophotographic equipment, it is conventional to employ a photoconductive plate in the form of a cylindrical drum which is continuously rotated through a cycle of sequential operations including charging, exposure, developing, transfer and cleaning. The plate is usually charged with corona with positive polarity by means of a corona generating device of the type disclosed by L. E. Walkup in U.S. Pat. No. 2,777,957 which is connected to a suitable source of high potential and the charged plate is then exposed to a light and shadow image to dissipate the charge on the areas exposed to the light thereby forming a latent electrostatic image. A developer comprised of toner loosely held on a coated carrier is conveyed from a reservoir to a point above the drum bearing the latent electrostatic image and is allowed to fall and roll by gravity over the image-bearing surface thereby forming a powder image. The carrier along with any unused toner is returned to the reservoir for recycle through the development system. After forming a powder image on the electrostatic image during the development step, the powder image is electrostatically transferred to a support surface by means of a corona generating device such as the corona device mentioned above. In automatic equipment employing a rotating drum, a support surface to which a powdered image is to be transferred is moved through the equipment at the same rate as the periphery of the drum and contacts the drum in the transfer position interposed between the drum surface and the corona generating device. Transfer is effected by the corona generating device which imparts an electrostatic charge to attract the powder image from the drum to the support surface. The polarity of charge required to effect image transfer is dependent upon the visual form of the original copy relative to the reproduction and the electroscopic characteristics of a developing material employed to effect development. For example, where a positive reproduction is to be made of a positive original, it is conventional to employ a positive polarity corona to effect transfer of a negatively charged toner image to the support surface. When a positive reproduction from a negative original is desired, it is conventional to employ a positive charged developing material which is repelled by the charged areas on the plate to the discharge areas thereon to form a positive image which may be transferred by negative polarity corona.
The coated carrier presently employed in development compositions perform quite satisfactorily, but the time period over which such satisfactory performance can be expected is severely limited by two factors; namely, impactation or mechanical flow and/or degradation. The impactation or mechanical flow is caused by the continuous tumbling of the carrier in the development system, and as a result, toner particles adhere to the carrier surface. As the amount of toner adhering to the carrier surface increases, the triboelectric properties of the carrier material are changed and eventually such changes result in unsatisfactory reproduction. Similarly, continuous use of the carrier produces wear and chipping of the resinous coating, exposing the core material, and the use of such an exposed carrier results in print deletion and poor print quality.
The primary factors limiting the life of the carrier, i.e., impactation and degradation, are a direct result of the defects inherent in the use of a resinous coating. The resinous coatings, however, provide the properties required for an effective carrier; namely, low humidity sensitivity, good triboelectric qualities, and density. The low humidity sensitivity is required in order to prevent changes in the triboelectric properties of the carrier with changes in ambient conditions. The triboelectric qualities of the carrier are important in that the mutual electrification of the carrier and toner, i.e., their relative positions in the triboelectric series, is a determining factor in the overall quality of the resulting print. The density of the carrier is an important factor in preventing adherence of the carrier to the image-bearing surface. Therefore, the limitations directly resulting from the use of a coated carrier are tolerated in order to provide the properties required for effective development.